Hard magnetic disks to be used as recording media in a computer system are tested for any surface defects and their size during fabrication at the stage of either a substrate or a finished disk having a magnetic coating (for the sake of convenience, both types are hereunder collectively referred to as "magnetic disks" or more simply as "disks").
Most of the disks in current use are 3.3 inches or less in diameter and their recording density has been increasing dramatically due to the use of GMR heads. These disks formerly used aluminum substrates but they now use glass disks having smaller heat expansion ratios and thicknesses in the order of 0.6-0.8 mm.
FIG. 4A shows the layout of the essential part of a conventional disk tester, or an apparatus for detecting surface defects on magnetic disks which is generally indicated by 10. The apparatus 10 consists of three major components, a rotating mechanism 2, a detecting optical system 3 and a surface defect detecting and processing unit 4. A disk 1 to be tested is mounted on a spindle 21 in the rotating mechanism 2 and driven to rotate with a motor (M) 22. The detecting optical system 3 has projection optics 31 comprising a laser light source 311 and a focusing lens 312. The laser beams L.sub.T from the light source 311 are focused by the lens 312 to form a spot S.sub.P that illuminates the surface of the disk 1.
If the disk 1 moves in the direction of X axis, the spot S.sub.P moves in the direction of the radius R of the disk 1 so that its surface is scanned spirally. In order to minimize the scan time, the spot S.sub.P takes on an elliptical shape having a minor axis .phi..sub.1 and a major axis .phi..sub.2 (see FIG. 4B). The major axis .phi..sub.2 is set perpendicular to the scanning direction so as to increase the scan width.
A defect F on the surface of the disk 1 scatters the light of spot S.sub.P. The scattered light S.sub.R is condensed by a condenser lens 321 in light receiving optics 32 and received by a light receiver 322 which is composed of photoelectric transducers such as avalanche photodiodes (APD) or photomultiplier tubes (PMT). An output signal from the light receiver 322 is input to a signal processing circuit 41 in the surface defect detecting and processing unit 4. The defect F is detected by the signal processing circuit 41 and its size is classified or calculated from the amplitude of said output signal. Being a circuit for detecting defects by a sampling technique, the signal processing circuit 41 comprises the following basic components: an amplifier for amplifying the output signal from the light receiver 322; a sampling circuit for detecting a peak defect value which, in response to pulses from a rotary encoder 23, samples peak values for those amplified output signals which represent defects that exceed noise; an A/D converter for digitizing the sampled peak values; and a position data generating circuit which generates data about position on the disk in response to pulses from the rotary encoder 23.
The data about the size of each defect and the data about its position on the disk are subjected to A/D conversion within the signal processing circuit 41 and thence input to a data processor 44 comprising an MPU 42, a memory 43, etc. In the data processor 44, the number of defects is counted for each size and the result is printed out by a printer (PR) 45 together with the position of each defect on the disk. Alternatively, the size of each defect is shown on the screen of a CRT 46 or other display together with its position on the disk. The count of defects of each size is also displayed as a separate piece of data.
The rotary encoder 23 is provided either adjacent to or in engagement with the rotating shaft of the motor 22. It detects the reference position for disk rotation and the amount of its rotation and sends the associated pulse signals to the signal processing circuit 41.
Defects F on the disk 1 can assume various shapes as exemplified in FIG. 5. Defect F.sub.h resembles a saucer having a diameter D.sub.h much greater than depth d.sub.h. Defect F.sub.P resembles awell having a diameter D.sub.P much smaller than depth d.sub.P and is commonly called a "pit". Both types of defects frequently occur as isolated phenomena. In contrast, F.sub.s is a linear defect called a "scratch" and its section can have various values of width w.sub.s and depth d.sub.s. Needless to say, the disk can have surface defects F of other shapes.
Aside from these defects in the form of a recess, burs or protrusions such as "extraneous substances" that typically result from the deposition of fine particles can occur in various sizes and heights.
In order to achieve satisfactory detection of defects of various shapes and sizes, the disk tester 10 provides optimum settings of detection sensitivity-related factors via a control panel 47 which include the angle of projection .theta..sub.T of laser beams L.sub.T from the projection optics 31, the angle of light reception .theta..sub.R by the light receiving optics 32, the voltage V to be applied to the light receiver 322 (APD), the gain of the built-in amplifier in the signal processing circuit 41, the threshold voltage E for noise rejection, and the laser output from the laser light source 311. To perform sensitivity adjustment, either an actual disk having sample defects (e.g. saucers, pits or scratches) of known sizes or an actual disk having burs of specified heights are used as a sample.
An invention based on this concept has been applied for patent by the Assignee under Unexamined Published Japanese Patent Application No. 325713/1998 entitled "Method and apparatus for detecting surface defects". According to this invention, testing for defects is performed using a sensitivity calibration disk having simulated defect rows of progressively varying sizes for high or low spots of irregularities and those simulated defect rows which progressively increase or decrease in size are radially displayed as the result of testing and the detection sensitivity is adjusted in accordance with the displayed test result.
The Assignee filed a United States patent application on a sensitivity calibration disk to be used by the detecting apparatus and U.S. Pat. No. 5,875,027 has issued, teaching the technology illustrated in the aforementioned FIGS. 4 and 5.
However, even such detection method or apparatus does not work satisfactorily if the disk to be tested is a glass disk having a thickness of about 0.6-0.8 mm. In response to vertical displacements of the disk, the light of laser beams L.sub.T scattered by the defects on the reverse surface of the disk or the extraneous substances deposited on it travels back to the obverse surface or the reflected light from the reverse surface becomes noise and, as a result, not only the precision in detection of scratches, nicks and burs on the glass surface is lowered but also the defects on the reverse surface of the disk are erroneously recognized as defects on the obverse surface.